The present invention relates to refrigeration systems and particularly refrigeration systems employed for air conditioning or climate control of the passenger compartment of an automotive vehicle. Automotive air conditioning systems typically employ an evaporator with a flow of blower air discharged over the evaporator into the passenger compartment for cooling. Refrigerant flow to the evaporator is through an expansion device or valve and is typically controlled by cycling an electric clutch for engaging the drive to the compressor.
Various expansion means may be employed to supply the refrigerant to the evaporator at a reduced pressure from the condenser. One technique employs a simple capillary tube; the second technique employs an expansion means in the form of a mechanically operated thermal expansion valve having a diaphragm responsive to changes in pressure in a closed chamber filled with refrigerant which is exposed to the temperature of the refrigerant discharging from the evaporator such that changes in temperature produce a change in pressure acting on the diaphragm for controlling flow through the valve. A third type of expansion means comprises an electrically operated valve typically having a solenoid controlled by an electronic controller utilizing a micro-computer for either proportional movement or modulated pulse movement. All of these techniques are known; examples of vehicle air conditioning systems controlled by mechanical expansion valves are shown in U.S. Pat. Nos. 4,794,762, 4,841,734, and 4,944,160. Examples of automotive air conditioning systems controlled by electrically operated expansion valves are shown and described in U.S. Pat. Nos. 4,790,145, 4,835,976, 4,835,976, 4,848,100, and 4,873,836.
In such systems where an electrically operated expansion valve is employed for controlling refrigerant flow to the evaporator, it is known to sense the evaporator discharge or suction return pressure and to provide an electrical signal indicative thereof to an electronic controller for generating a signal to cut off the compressor clutch when the suction return pressure falls below a predetermined level. In systems of this latter type, when the thermal load on the evaporator is high, e.g., when the interior of the vehicle is very hot, it is desirable to run the evaporator as cold as possible to effect a maximum rate of cool down for the passenger compartment. Under such conditions, it is desired to maintain the compressor energized or operable on an uninterrupted basis so long as there is no likelihood of condensate freezing and ice formation on the exterior of the evaporator. Under conditions of high thermal load, it is undesirable to cycle the compressor "OFF" because of the attendant rise in blower discharge air over the evaporator which reduces the rate of cooling of the passenger compartment interior.
However, if the compressor is allowed to run continuously and the evaporator is maintained as cold as possible under conditions of moderate thermal loading, ice may form on the evaporator fins blocking off air flow, resulting in evaporator freeze-up. Thus, the compressor clutch must be cycled "OFF" before freeze-up occurs.
Heretofore, in air conditioning systems employing expansion valves and particularly systems having electrically operated expansion valves operated by an electronic controller, the suction pressure sensor provides a signal to the controller which cycles the compressor clutch "OFF" when the evaporator discharge or suction pressure drops below a predetermined level.
However, it has been found that under maximum thermal loading conditions it is desirable to provide a lower evaporator discharge or suction pressure cut-out setting for the compressor clutch. Therefore, it has been desired to find some way or means of controlling the compressor clutch in order to effect maximum cool-down under high thermal load conditions, yet to prevent evaporator freeze-up during moderate or low thermal load conditions.